Earthquakes are caused primarily by the sudden release of stored elastic strain energy in the Earth's crust or upper mantle. The most common mechanism is sudden slip along a fault, which is a fracture or zone of fractures where rocks have moved relative to one another. Tectonic plates are continuously moving, but friction and the strength of rocks can prevent immediate movement along locked portions of faults. As tectonic stress accumulates, rocks deform elastically.
When the stress exceeds the strength of the fault system or conditions allow frictional resistance to be overcome, the fault can rupture and slip rapidly. The resulting release of elastic energy travels outward as seismic waves. Most earthquakes are therefore tectonic in origin, although earthquakes can also be caused by volcanic activity, landslides, mining, reservoir loading and human activities that alter underground stresses or fluid pressures.
Plate tectonics provides the broadest explanation for the distribution of earthquakes. Earthquakes occur frequently along convergent, divergent and transform plate boundaries. At convergent boundaries, plates move toward one another and can generate powerful earthquakes through subduction or continental collision. At divergent boundaries, plates move apart and normal faulting can occur as the crust is extended. At transform boundaries, plates slide laterally past each other and strike slip earthquakes can occur.
Earthquakes also occur within plate interiors when older faults are reactivated or stresses are transmitted through the lithosphere. Consequently, an earthquake does not require a plate boundary to exist directly beneath the epicentre. Some intraplate earthquakes can occur far from active plate margins, although they are generally less frequent than earthquakes in highly active tectonic zones.
Fault type strongly influences how rocks move during an earthquake. Normal faults occur where the crust is extended and one block moves downward relative to another. Reverse faults occur under compression and involve upward movement of one block relative to another. A thrust fault is a low angle reverse fault and can produce exceptionally large earthquakes when extensive sections of a subduction interface rupture.
Strike slip faults involve predominantly horizontal movement and can be either right lateral or left lateral depending on the relative direction of displacement. Oblique slip combines vertical and horizontal movement. These categories describe the geometry of fault motion and help geologists reconstruct the stresses responsible for earthquakes. The amount of slip, rupture area, rock rigidity and distribution of displacement all influence the energy released.
Friction plays a central role in earthquake generation. Fault surfaces are not perfectly smooth, and contact between rocks can resist movement. Frictional resistance can temporarily lock a fault while tectonic forces continue to load it. The rocks surrounding the fault deform and store elastic energy. When rupture begins, the fault can propagate rapidly across a substantial area. The rupture velocity is typically a significant fraction of the speed of seismic waves through the surrounding rock.
The earthquake ends when the available stress and energy are sufficiently reduced or when the rupture encounters conditions that stop further propagation. Aftershocks can occur as the crust adjusts to the changed stress distribution. This process explains why earthquakes are commonly followed by numerous smaller earthquakes, although aftershock sequences vary considerably between events.
Earthquakes can also result from volcanic processes. Magma moving through the crust changes pressure and can fracture surrounding rock, producing volcanic earthquakes. Harmonic tremor and swarms of small earthquakes can accompany magma movement and are important signals in volcano monitoring. Earthquakes can also be induced or triggered by human activities. Fluid injection into deep wells can increase pore pressure along pre existing faults, reducing the effective frictional resistance that keeps the fault locked.
Reservoir impoundment, geothermal operations, mining and underground fluid extraction can also alter stresses. Human induced seismicity is generally smaller than the largest tectonic earthquakes, but some induced events can be damaging. Establishing causation requires geological, temporal, hydraulic and statistical evidence rather than simply observing that an earthquake occurred near an industrial activity.
The depth and location of an earthquake depend on the tectonic environment and the physical conditions within the crust and upper mantle. Shallow earthquakes occur within the upper part of the lithosphere and can be particularly damaging because their energy is released close to populated areas. Intermediate and deep earthquakes occur mainly within subducting slabs, where cold oceanic lithosphere can remain mechanically capable of brittle or earthquake generating deformation to depths of several hundred kilometres.
Earthquakes do not occur at arbitrary depths because increasing temperature and pressure generally promote ductile deformation rather than brittle failure, although subducting slabs create exceptional conditions. The depth of an earthquake is therefore an important parameter for understanding its tectonic origin and potential surface effects.
Earthquake damage is influenced by far more than the earthquake's magnitude. Ground shaking depends on the earthquake's size, depth, distance, rupture direction, local geology and wave frequencies. Soft sediments can amplify certain frequencies of ground motion compared with hard bedrock. Basin geometry can prolong shaking, while steep slopes can increase landslide risk. Buildings and infrastructure respond differently depending on construction, height, materials, foundation conditions and engineering standards. Secondary hazards can include liquefaction, landslides, fires, tsunamis, dam failures and ground rupture. Consequently, a smaller shallow earthquake beneath a densely populated urban area can produce greater damage than a much larger earthquake that occurs deep beneath a sparsely populated region.
The fundamental cause of a tectonic earthquake is therefore the interaction between stress, rock strength, friction and sudden fault rupture. Tectonic plates supply the long term deformation and stress, while individual faults provide the locations where strain can accumulate and be released. The elastic rebound concept explains that rocks on either side of a fault can deform before an earthquake and partially return toward a less strained configuration after rupture.
Earthquake science combines seismology, geology, geodesy, satellite measurements and numerical modelling to understand these processes. Scientists can identify active faults and estimate long term earthquake hazards, but the exact time, place and magnitude of a future earthquake generally cannot be predicted reliably from observations alone.
